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Geotechnical Engineering Centre CRICOS Provider No 00025B THE MYSTERY OF UNSATURATED SOIL MECHANICS – SOME MINING APPLICATIONS Australian Geomechanics.

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Presentation on theme: "Geotechnical Engineering Centre CRICOS Provider No 00025B THE MYSTERY OF UNSATURATED SOIL MECHANICS – SOME MINING APPLICATIONS Australian Geomechanics."— Presentation transcript:

1 Geotechnical Engineering Centre CRICOS Provider No 00025B THE MYSTERY OF UNSATURATED SOIL MECHANICS – SOME MINING APPLICATIONS Australian Geomechanics Society, SA & NT Chapter Monday 15 July 2013, Adeliade Professor David J Williams

2 Geotechnical Engineering Centre CRICOS Provider No 00025B Overview Unsaturated soil mechanics continues to play poor relation to saturated soil mechanics An unsaturated soil at a given density performs better than same soil in a saturated state Some mining applications: –Wetting-up and drain down of waste rock dumps –Drain down, desiccation and rewetting of mine tailings –Drain down, desiccation and re-wetting of product coal –Bearing capacity and deformation of mine wastes –Shear strength and compressibility of clay-rich spoil –Performance of geo-covers placed on mine wastes 2

3 Geotechnical Engineering Centre CRICOS Provider No 00025B INTRODUCTION

4 Geotechnical Engineering Centre CRICOS Provider No 00025B Key Unsaturated Soil Mechanics Parameters 1.Shear strength: –Or capacity of a soil to support load –Simplistically, shear strength can be tested under unsaturated conditions (adding “cohesion”) 2.Compressibility: –Or deformation of a soil under an applied load –Simplistically, compressibility can be tested under unsaturated conditions 3.Permeability (hydraulic conductivity): –Or rate of drainage under an applied load –Assessed by SWCC and k sat testing 4 INCREASED DECREASED

5 Geotechnical Engineering Centre CRICOS Provider No 00025B Mining and Mineral Processing Wastes Coarse-grained wastes (surface dumps): –Overburden (typically up to 200 mm) or waste rock (up to 1 m), particularly from surface mining (limited from underground mining) –Coarse reject (typically 50 mm) from coal processing –Slag or scats (typically 15 mm) from smelting –Spent heap leach material (typically 15 mm) Fine-grained wastes (tailings storage facilities): –Crushed and ground tailings (typically silt-size) –Erosion sediments (fines) 5

6 Geotechnical Engineering Centre CRICOS Provider No 00025B Waste Rock and Coarse Processing Waste Dumps 6 Waste rock dump Pumped co-disposal of coal washery wastes Coarse reject dump

7 Geotechnical Engineering Centre CRICOS Provider No 00025B Tailings Storage Facilities 7 Metalliferous tailings Iron ore tailings Coal tailings Red mud Sand mining tailings In-pit coal tailings

8 Geotechnical Engineering Centre CRICOS Provider No 00025B WETTING UP AND DRAIN DOWN OF WASTE ROCK DUMPS

9 Geotechnical Engineering Centre CRICOS Provider No 00025B Impact of Waste Rock Wetting-Up 9 Potential impact of waste rock dump wetting-up Average annual rainfall ~600 mm

10 Geotechnical Engineering Centre CRICOS Provider No 00025B Wetting-Up and Drain Down of Coarse-Grained Wastes min ~60 mm 25 min ~30 mm 80 min ~100 mm Start-up

11 Geotechnical Engineering Centre CRICOS Provider No 00025B Moisture Profile after 100 mm 11

12 Geotechnical Engineering Centre CRICOS Provider No 00025B Surface Infiltration into Cadia’s Trial Waste Rock Dump 12

13 Geotechnical Engineering Centre CRICOS Provider No 00025B Changing Infiltration into Cadia’s Trial Waste Rock Dump with Time November 2007 May 2006 Note differential settlement- induced ponding 13

14 Geotechnical Engineering Centre CRICOS Provider No 00025B Average Base Seepage Beneath Top and Side Slopes of Cadia’s TWRD 14

15 Geotechnical Engineering Centre CRICOS Provider No 00025B Trigger Rainfall and Delay for Base Seepage Following Rainfall 15

16 Geotechnical Engineering Centre CRICOS Provider No 00025B Estimated Wetting-Up and Continuum Breakthrough 16 Continuum breakthrough at: ~25% saturated for fresh WR ~60% saturated for weathered WR

17 Geotechnical Engineering Centre CRICOS Provider No 00025B Estimated Time to “Continuum Breakthrough” of Waste Rock Dumps Increasing rainfall Increasing time Increasing dump height 17 Typical mine life ~20 years, during which time WRD is likely to be uncovered

18 Geotechnical Engineering Centre CRICOS Provider No 00025B DRAIN DOWN, DESICCATION AND REWETTING OF MINE TAILINGS

19 Geotechnical Engineering Centre CRICOS Provider No 00025B Impact of Tailings Slurry Deposition 19 Average annual rainfall ~600 mm Assuming 0.5 m/year rate of rise at 25% solids or 1 m/year at 50% solids

20 Geotechnical Engineering Centre CRICOS Provider No 00025B Mt Keith Tailings Laboratory Column Test 20

21 Geotechnical Engineering Centre CRICOS Provider No 00025B Comparison of Field and Laboratory SWCC Data for Mt Keith Tailings 21

22 Geotechnical Engineering Centre CRICOS Provider No 00025B Comparison Between Field and Laboratory Hydraulic Conductivities 22

23 Geotechnical Engineering Centre CRICOS Provider No 00025B Sensors on Towers in Trial TSF Cell Piezometer TDR moisture sensor Matric suction sensor 23

24 Geotechnical Engineering Centre CRICOS Provider No 00025B Deposition in Trial TSF Cell 24

25 Geotechnical Engineering Centre CRICOS Provider No 00025B 25 Matric Suction vs. Time 25

26 Geotechnical Engineering Centre CRICOS Provider No 00025B 26 Matric Suction vs. Depth 26

27 Geotechnical Engineering Centre CRICOS Provider No 00025B DRAIN-DOWN, DESICCATION AND RE- WETTING OF PRODUCT COAL

28 Geotechnical Engineering Centre CRICOS Provider No 00025B Dewatering and Atmospheric Effects on Product Coal 28 Coarse Fine Ultra-Fine Field stockpile 11.5 m 38 o

29 Geotechnical Engineering Centre CRICOS Provider No 00025B SWCCs of Product Coal Size Fractions 29

30 Geotechnical Engineering Centre CRICOS Provider No 00025B Duration of Applied Pressure – Centrifuging of Fine Product Coal 30

31 Geotechnical Engineering Centre CRICOS Provider No 00025B Duration of Applied Pressure – Vacuum Filtration of Ultra-Fine Coal 31

32 Geotechnical Engineering Centre CRICOS Provider No 00025B Vacuum filtration vs. Briquetting of Ultra-Fine Product Coal 32

33 Geotechnical Engineering Centre CRICOS Provider No 00025B Sampling Field Coal Stockpile for PSD and Moisture Content 33

34 Geotechnical Engineering Centre CRICOS Provider No 00025B Wetting-Up of Model Coal Stockpile 34

35 Geotechnical Engineering Centre CRICOS Provider No 00025B Stockpile Total Moisture Content Distribution 35

36 Geotechnical Engineering Centre CRICOS Provider No 00025B BEARING CAPACITY AND DEFORMATION OF MINE WASTES

37 Geotechnical Engineering Centre CRICOS Provider No 00025B Tailings Bearing Capacity Issues 1.5 m “Bow-wave” 4-5 m Too thin a cover over soft tailings “Bow-waving” of crusted tailings 37

38 Geotechnical Engineering Centre CRICOS Provider No 00025B Covering Wet Tailings –Victorian Gold Mine 38 Surcharging edge of tailings Spreading thin cover D6 dozer Ponded water due to drainage of excess pore water pressure 2-3 m surcharge 1 m cover, avoiding “bow wave” failure

39 Geotechnical Engineering Centre CRICOS Provider No 00025B Shear Strength Profiles of Coal Tailings 39

40 Geotechnical Engineering Centre CRICOS Provider No 00025B Safe Height of Fill on Coal Tailings 40 H = N c.s v /F.  ~5.14.s v /(3 x 18) ~0.095 s v

41 Geotechnical Engineering Centre CRICOS Provider No 00025B SHEAR STRENGTH AND COMPRESSIBILITY OF CLAY-RICH SPOIL

42 Geotechnical Engineering Centre CRICOS Provider No 00025B Jeebropilly Spoil – Relative Unscalped PSDs CLAY Weathered Rock Unweathered coarse-grained Rock Unweathered fine-grained Rock 42

43 Geotechnical Engineering Centre CRICOS Provider No 00025B Mt Arthur Spoil – Relative Unscalped PSDs 43

44 Geotechnical Engineering Centre CRICOS Provider No 00025B Gravimetric Moisture Content vs. Matric Suction Jeebropilly rocky spoil is less dense and wetter than Hunter Valley spoil, although similar degree of saturation, and hence matric suction 44

45 Geotechnical Engineering Centre CRICOS Provider No 00025B Jeebropilly Weathered Rock PSDs 45

46 Geotechnical Engineering Centre CRICOS Provider No 00025B Comparison of Direct Shear Friction Angles (-2.36 mm) 46 Note substantial softening on wetting up of clay-rich Jeebropilly Spoil

47 Geotechnical Engineering Centre CRICOS Provider No 00025B UPC 150 mm, 10 MPa Consolidometer 47

48 Geotechnical Engineering Centre CRICOS Provider No 00025B 76 & 150 mm Consolidometer Testing of Jeebropilly Weathered Rock Note low loose density and substantial consolidation on loading and wetting of clay-rich Jeebropilly Spoil 48

49 Geotechnical Engineering Centre CRICOS Provider No 00025B 76 & 150 mm Consolidometer Testing of 3-Month Old Mt Arthur Sandstone Note high loose density and limited consolidation on loading and wetting of Mt Arthur Sandstone 49

50 Geotechnical Engineering Centre CRICOS Provider No 00025B Large Texture Changes on Weathering of Jeebropilly Weathered Rock (-19 mm) 0 days8 days (32.6 mm rain) 21 days (+6.8 mm rain)15 days (+23.2 mm rain) 50

51 Geotechnical Engineering Centre CRICOS Provider No 00025B Large PSD Changes on Weathering of Jeebropilly Weathered Rock (-19 mm) 51

52 Geotechnical Engineering Centre CRICOS Provider No 00025B Settlement of Jeebropilly Weathered Rock with Time on Weathering (-19 mm) 52

53 Geotechnical Engineering Centre CRICOS Provider No 00025B Minimal Texture Changes on Weathering of Mt Arthur Sandstone (-19 mm) 0 days 7 days (4.0 mm rain) 14 days (+39.2 mm rain) 28 days (+6.8 mm rain)21 days (+47.8 mm rain) 53

54 Geotechnical Engineering Centre CRICOS Provider No 00025B PERFORMANCE OF STORE AND RELEASE COVERS PLACED ON MINE WASTES

55 Geotechnical Engineering Centre CRICOS Provider No 00025B Influence of Climate – Natural Recharge (Beekmann et al., 1996) 55 ARID SEMI ARID Av. rainfall ~600 mmpa ~70 mm ~2.2 x m/s

56 Geotechnical Engineering Centre CRICOS Provider No 00025B Store and Release Cover on Mine Wastes 56

57 Geotechnical Engineering Centre CRICOS Provider No 00025B Construction of Store and Release Cover on Kidston’s Waste Rock Dumps 57 Rocky soil mulch, capable of storing up to 300 mm rainfall/m thickness Compacted sealing layer

58 Geotechnical Engineering Centre CRICOS Provider No 00025B Vegetation of Kidston’s Store and Release Cover month old trees (no grasses) – December month old trees & 6-month old grasses – July year old trees and 1-year old grasses

59 Geotechnical Engineering Centre CRICOS Provider No 00025B 59

60 Geotechnical Engineering Centre CRICOS Provider No 00025B Up to 2-week lag k ~2 x m/s 60

61 Geotechnical Engineering Centre CRICOS Provider No 00025B Kidston – Field SWCC Data and Fitted Curves Unreliable 61 Desiccated surface Wet sealing layer

62 Geotechnical Engineering Centre CRICOS Provider No 00025B Store and Release Net Percolation 62

63 Geotechnical Engineering Centre CRICOS Provider No 00025B Mt Whaleback Store and Release Cover (O’Kane et al, 2012) CRICOS Provider No 00025B 63 Note: No base sealing layer and constructed with coarse-grained ROM waste rock, i.e. poorly- constructed

64 Geotechnical Engineering Centre CRICOS Provider No 00025B 64 Mt Whaleback Store and Release Cover (O’Kane et al, 2012) Note: A well-constructed store and release cover should store ~250 mm/m before percolating

65 Geotechnical Engineering Centre CRICOS Provider No 00025B Kidston – Schematic Wetting-Up and Drain Down with Time INFILTRATION/SEEPAGE TIME (years) During operation Post covering Rainfall infiltration, diminishing exponentially as surface compacts and with cover Base seepage, increasing as dump wets-up, diminishing exponentially with cover due to drain down Final seepage = net percolation ~7 mm/year, on average

66 Geotechnical Engineering Centre CRICOS Provider No 00025B NEW GEOTECHNICAL ENGINEERING CENTRE AT THE UNIVERSITY OF QUEENSLAND

67 Geotechnical Engineering Centre CRICOS Provider No 00025B New Geotechnical Engineering UQ Commenced February 2012 Funded by Golder Associates, Rio Tinto, AngloGold Ashanti and BHP Billiton, in partnership with UQ –Industry Partners each contributing ~$150,000/year –Matching funding from UQ –Total funding of $6 million over 5 years –3 new academic appointments, making a total of 7: Professor of Rock Mechanics (to be appointed) Lecturer in Rock Mechanics (Dr Nazife Erarslan) Lecturer in Hydrogeology (offered) –Plus supporting PostDocs 67

68 Geotechnical Engineering Centre CRICOS Provider No 00025B The GEC Vision New Civil & Geotechnical Engineering and Mining & Geotechnical Engineering Dual Major Programs will produce: –Civil and Mining Engineering graduates with a specialisation in Geotechnical Engineering, unique in Australia! (Canada has 5 or 6 such programs, e.g., making their graduates much sought after by Australian companies) –Graduates in high demand from Geotechnical and Mining Engineering Consultants, Mining Companies, and Civil and Mining Contractors –New Programs will have graduates/year –Collaborative research with our industry partners 68

69 Geotechnical Engineering Centre CRICOS Provider No 00025B Civil & Geotechnical Engineering 4-Year Program 2 (6%) Soil Mechanics Courses → 10 (31%) Geotechnical Engineering Courses (in Green) EXISTING NEW 69

70 Geotechnical Engineering Centre CRICOS Provider No 00025B Mining & Geotechnical Engineering 4-Year Program 5 (15%) Mining Geomechanics Courses → 12 (38%) Geotechnical Engineering Courses (in Green) EXISTING NEW 70

71 Geotechnical Engineering Centre CRICOS Provider No 00025B Expected Geotechnical Engineering Graduations 71

72 Geotechnical Engineering Centre CRICOS Provider No 00025B Postgraduate Research Students PhD students in Geotechnical Engineering: –Currently 20 –> 5 applications pending Research topics include: –Mine tailings water cycle –Effectiveness of liners for water storage ponds –Settlement of high coal mine spoil –Flow in coal seams –Behaviour of screw auger piles in clay –Problematic clay-rich coal mine tailings –Mining below the groundwater table –Erosion of granular structures –Computational geomechanics –Development and application of spatial Time Domain Reflectrometry 72


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